A recent long-distance optical transmission system used for high-speed large-capacity data transmission adopts a Wavelength Division Multiplex (WDM) method, in which a number of optical signals are multiplexed on a single transmission path. In WDM communication, optical lights having different wavelengths are simultaneously transmitted.
Besides, there is proposed a method of polarization multiplexing communication in which polarizations having different polarization orientations are multiplexed (e.g., see Japanese Laid-open Patent Publication No. 2010-068235). Polarization multiplexing communication maintains multiplicity of horizontal polarization and vertical polarization, keeping each individual orientation, so that an amount of information transmitted via an optical signal of a single wavelength light becomes twice. In other words, application of the optical polarization multiplexing technique to optical signals of respective wavelength lights to be transmitted optical wavelength multiplexing communication can increase an amount of communicable information per wavelength. Thereby, the transmission capacity per fiber can be increased.
A polarization multiplexed signal conforming to the polarization multiplexing communication is demultiplexed by, for example, adaptive polarization demultiplexing signal processing using analog/digital conversion or digital signal processing conforming to digital coherent method. Adaptive polarization demultiplexing signal processing adaptively estimates a property of polarization of a receiving signal and demodulates polarization multiplexed signal in a digital circuit. Specific demodulation methods are splitting of polarizations, equalizing of wavelength distortion, compensating for frequency offset, and controlling of the phase of a signal.
An optical signal is attenuated in proportional to a length that the signal travels through the transmission path. That is, in transmission through a long-distance transmission path such as a submarine cable, an optical signal is largely attenuated. Amplification on the transmission path is effectual to keep signal intensity. For this purpose, a number of optical amplifiers are serially interposed into a transmission path in a conventional long-distance optical transmission system.
However, connection of multiple optical amplifiers result in lowering signal quality. In particular, Polarization Hole Burning (PHB) causes each optical amplifier to have a Polarization Dependent Gain (PDG). Consequently, spontaneous emission light serving as a noise component in an optical amplifier increases in relation to the gain so that the signal-to-noise (SN) ratio of an optical signal output from the optical amplifier lowers.
The intensity of a signal output from an optical amplifier have a Polarization Dependent Loss (PDL) slightly related to the polarization of the optical signal. In the meantime, the noise component in the form of spontaneous emission light is not polarized. Therefore, the ratio of noise in an optical signal relatively increases at the downstream part of an optical amplifier, and accordingly, the SN ratio further lowers. The noise component is accumulated each time an optical signal passes through an optical amplifier and therefore the SN ratio lowers each time the optical signal passes through an amplifier.
In the above polarization multiplexing communication, optical signals of respective different wavelengths do not interfere with one another and are multiplexed into a polarization multiplexed signal, which scarcely causes PHB. However, PDL of each optical device disposed on the transmission path may increase variation in optical power of respective polarization different in polarization orientation and consequently PHB may occur.
In order to avoid influence caused by PHB and PDL, Polarization Scrambling technique is known which polarization is forcibly modulated when the corresponding optical signal is to be transmitted (see, for example, Japanese Laid-open Patent Publication No. 11-271698). This technique obtains the similar effects as non-polarized light through randomly varying the state of polarization of an optical signal and is effective in improving an SN ratio of an optical signal.
In polarization scrambling, a lower frequency of modulating polarization results in higher digital error rate penalty. Generally, a modulation frequency set to be 10 kHz or less than 10 kHz causes the optical intensity to deviate at a frequency of 10 kHz or less than 10 kHz due to PDL. Furthermore, modulation of a gain resulting from the frequency response property of an optical modulator leads to a larger intensity modulation, which greatly impairs the signal quality. For this reason, when polarization scrambling is carried out on an optical transmission path, the polarization is preferably modulated at a frequency of several hundreds or more.
However, in applying adaptive polarization demultiplexing signal processing conforming to a digital coherent method to the above polarization multiplexing communication, hardware restriction of a digital circuit makes it difficult to increase the modulation frequency in polarization scrambling.
Accordingly, in a long-distance optical transmission system mounting adaptive polarization splitting signals processing conforming to a digital coherent method, polarization scrambling is carried out at relatively low modulation frequency so that there is difficulty in improving transmission quality. Other long-distance transmission systems have the similar problem.